Image information, be it color or black and white, is commonly derived by scanning, initially at least, in a grey level format containing a large number of levels, e.g.: 256 levels for black and white and more than 16 million (256.sup.3) levels for color. This multi-level format is usually unprintable by standard printers.
The term "grey level" is used to described such data for both black and white and color applications. Standard printers print in a limited number of levels, either a spot or a no spot in the binary case, or a limited number of levels associated with the spot, for example, four in the quaternary case. Since grey level image data may be represented by very large values, it is necessary to reduce grey level image data to a limited number of levels so that it is printable. Besides grey level image information derived by scanning, certain processing techniques, such as computer generation, produce grey level pixel values which require such a conversion.
One standard method of converting grey level pixel image data to binary level pixel image data is through the use of screening, dithering, or halftoning. In such arrangements, over a given area, each grey level pixel within the area is compared to one of a set of distinct preselected thresholds. The set of thresholds comprises a matrix of threshold values or a halftone cell.
In a typical circuit, an unmodified image or video signal is fed into a modulation circuit with a screen value from a halftone screen matrix to produce a modified signal. The modified signal is then thresholded by a binarization circuit to produce a binary output. The binary output represents either the ON or OFF characteristic of the processed pixel. It is noted that the screen could be developed so as to replace the threshold value such that the threshold value would change from pixel to pixel and the system would not require the adding of the screen value before thresholding. These are equivalent systems. For a fixed video signal V, the screen modulated video signal V.sub.s ' has values varying between the levels A and B as the screen value S vary between 255 and 0. Thus, the effective white and black values to be used in the binarization process or calculation should be, for example, for the value of white, 0 and, for the value of black, 255.
In the described process, the sampled image picture elements are compared with a single threshold, and a black/white decision is made. However, the threshold relationship is modified by modulating the image data with the screen data. The screen data is selected in sequential order from a two-dimensional matrix defined as a halftone cell threshold set. The set of screen values and the arrangement therein determine the grey scale range, frequency, angle, and other properties of the halftone pictorial image.
The effect of such an arrangement is that, for an area where the image is grey, some of the thresholds within the matrix will be exceeded, while others are not. In the binary case, the portions of the matrix, or cell elements, in which the thresholds are exceeded are printed as white, while the remaining elements are allowed to remain black or vice-versa depending on the orientation of the system, (write white system or write black system). For example, 255 may represent white in one system, (write white), but black in another system, (write black). The effect of the distribution of black and white over the cell is integrated by the human eye as grey.
However, typical screening presents problems in that the amount of grey within an original image is not maintained exactly over an area because the finite number of elements inside each halftone cell only allows the reproduction of a finite number of grey levels. The error arising from the difference between the threshold value and the actual grey level value at any particular cell is, conventionally, thrown away. This results in loss of image information and creates significant image artifacts, such as banding or false contours that can be seen in smooth image areas. In banding, the image input grey level varies smoothly over an area while the halftoned image has to make a transition from one halftone dot (grey level) to another. This transition can clearly be seen as a band or contour running through smooth image parts.
Another problem associated with screening grey images is the trade-off between the screen frequency and the number of grey levels available. Although it is desirable to use a high frequency screen, the number of grey levels available decreases as the screen frequency increases. One method which has been proposed to increase the number of grey levels as the screen frequency increases is set forth in U.S. Pat. No. 5,317,653 to Eschbach et al. The entire contents of U.S. Pat. No. 5,317,653 are hereby incorporated by reference.
In this method, the grey image is first reduced to a small number of grey levels with error diffusion, and then a line screen with a small number of grey levels and a high frequency is used. Any errors from the screening process are discarded. This two step process binarizes the image.
However, to implement such a method, a print engine or system would require a multi-level error diffusion process followed by screen thresholding. Moreover, the error from the screening process is not diffused, and thus, banding and the other described artifacts may still form. Also, any screening process must be able to exploit the fill dynamic range of the image processing system. Therefore, it is desirable to provide screening, but without departing from the typical image processing architecture of printing system, which significantly reduces the image artifacts and exploits the full dynamic range.